The cell contents of a primary (non-rechargeable) alkaline cell typically contain an anode comprising zinc, alkaline electrolyte, a cathode comprising manganese dioxide, and an electrolyte permeable separator film between the anode and cathode. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cell contents are typically housed in a cylindrical steel container. The anode material comprises zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte solution. The gelling agent holds the zinc particles in place and in contact with each other. The cathode material comprises manganese dioxide, and small amount of electrolyte and may also include small amounts of carbon or graphite to increase conductivity. The cathode material is a solid material compressed against the inside surface of the cell casing forming a hard compacted mass. An ion porous separator material, typically of cellulosic material, is placed over the inside surface of the cathode. The anode material is typically inserted into the core of the casing with the anode and cathode separated by the separator material. A conductive metal nail, known as the anode current collector, is typically inserted into the anode material and is in electrical contact with an end plate which forms the cell's negative terminal.
There is a growing need to make primary alkaline cells better suitable for high power application. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices require high current drain rates of between about 0.5 and 2 Amp, typically between about 0.5 and 1.5 Amp. Correspondingly, they require operation at power demands between about 0.5 and 2 Watt.
Conventional alkaline cells have solid cathodes comprising particulate manganese dioxide. Electrolytic MnO.sub.2 (EMD) is generally preferred because of its high density and since it is conveniently obtained at high purity by electrolytic methods. Other particulate MnO.sub.2, for example, MnO.sub.2 obtained by chemical methods referred to in the art as chemical MnO.sub.2 (CMD) may also be used. Such chemical MnO.sub.2 (CMD) is generally less dense than electrolytic MnO.sub.2 (EMD). In conventional alkaline cell cathodes the manganese dioxide composition is high--between about 70 and 87 percent by weight. Conventional alkaline cell cathode may typically be composed of electrolytic manganese dioxide (80-87 wt %), graphite (7-10 wt %), and a 7-11 Normal "aqueous KOH solution" (5-7 wt %). Such mixtures form a moist solid mix which is fully compacted into the cell casing using plungers or other such compacting devices forming a compacted solid cathode mass. The cathode material may be preformed into the shape of pellets or rings which are inserted into the cell in stacked arrangement, for example, as shown in U.S. Pat. No. 5,283,139, and then recompacted. The resulting compacted cathode in either case is a hard, solid material.
U.S. Pat. No. 5,501,924 discloses such conventional solid cathodes comprising MnO.sub.2 for alkaline cell. For example, this reference discloses a cathode for a D size alkaline cell wherein the cathode is preferably composed of 71.7 to 81.7 weight percent MnO.sub.2, about 8.5 weight percent graphite, and about 7.9 weight percent alkaline solution, such as 45% KOH solution, and about 0.4 weight percent deionized water, and about 1.5 weight percent binder solution containing tetrafluoroethylene in water. The cathode also contains about 0.1 to 10 weight percent SnO.sub.2 additive such that the combined weight percent of MnO.sub.2 and SnO.sub.2 is a constant of preferably about 81.8. (col. 2, lines 46 to 58). The reference discloses that such cathode material is placed into a steel can and molded into shape. The reference also discloses a cathode for AA size alkaline cell wherein the cathode is composed of 74.0 to 84.0 weight percent MnO.sub.2, about 7.3 weight percent graphite, and about 7.2 weight percent alkaline solution, such as 45% KOH solution, and about 1.5 weight percent deionized water and about 0.1 to 10 weight percent SnO.sub.2 additive such that the combined weight percent of the MnO.sub.2 and the SnO.sub.2 is preferably about 84.0. (col. 2, line 63 to col. 3, line 6) A person skilled in the art would recognize that the recited cathode material for either the D or AA cell would be a solid loosely packed mix of material even before it is placed in the cell because of the high solids content and low liquid content. Once molded in the cell this mass becomes even further compacted to form a compacted solid cathode mass because of the high level MnO2 and solids content in conjunction with the relatively low liquid content. Such mass has the property that it readily becomes compacted into a rigid solid structure upon application of compressive forces.
U.S. Pat. No. 5,489,493 discloses that the porosity of the cathode comprising MnO.sub.2 for alkaline cells may be adjusted by admixing a high porosity manganese dioxide (CMD) with a low porosity manganese dioxide (EMD). The reference discloses that when the materials are combined in a homogeneous mixture the preferred average porosity of the cathode is 15% to 35% and more preferably 20% to 25%. (col. 4, lines 25-29.) The reference discloses that the cathode material typically comprises between bout 80 to 85 weight percent of the total cathode and that an amount of electrolyte solution is added sufficient to wet the dry components. The mix is molded or compressed against the container, or premolded as rings and the rings pressed into the container. (col. 5, lines 28-30) A person skilled in the art would recognize that the referenced cathode material molded or compressed against the container results in a compacted solid mass of material because the reference indicates that the cells had a high (73.2 volume percent) solid packing in the cathode (col. 2, line 57-59).
The strategy in formulating conventional alkaline cell cathode is to employ high concentration of manganese dioxide in the mix in order to achieve high capacity (amp-hrs). In high power application, that is, at high current drain, deleterious effects such as cathode polarization can occur. Polarization results from limited mobility of ions within the cathode active material and within the electrolyte, which in turn reduces service life. The phenomenon of cathode polarization may be caused by the accumulation of hydroxide ions in the cathode during high rate discharge. The accumulation of hydroxide ions in the cathode prevents these ions from reaching the anode where they are required to sustain the oxidation of zinc, that is, to react with zinc ions to form zinc oxide by-product. In conventional solid MnO.sub.2 cathodes the polarization effect may be so serious at high current drain, for example, between about 1 and 1.5 Amp drain that only 20% or less of the cell's theoretical capacity (Amp-hr) may be obtainable. Thus, it is desirable to provide a way of reliably increasing the manganese dioxide utilization (actual specific capacity of the manganese dioxide, Amp-hr/g) at high drain without adversely affecting cell performance.
Also, conventional MnO.sub.2 cathodes for alkaline cells, because they are solid material, have the disadvantage that they are susceptible to breakage during compaction or accidental cell impact after the cathodes have been compacted into the cell. Also, tools are required to compact the solid cathodes into the cell. These tools need to be replaced frequently because of wear as they are constantly applied against the hard solid cathode during mass production cell assembly.
Thus, it is desirable to avoid cathode breakage and the need for replacement tools to compact the cathode in the cell.